PHYSICS 119 



between lines. This success is the more remarkable as Ritz's principle is almost purely 

 empirical. 



Koenigsberger 1 has worked out a theoretical formula for rinding the proportion of 

 molecules taking part in any emission or absorption of light. He finds that only about 

 one molecule in a thousand is active at any given instant. While this result is in 

 agreement with estimates previously based on other data, an outstanding spectroscopic 

 difficulty is still presented by the shifting of spectrum lines with a variation in the pressure 

 of the gas in which the source is immersed. This effect, discovered by Humphreys and 

 Mohler, has since been studied by Duffield, Rossi, King and others, but all that is clear 

 up to the present is that this effect has no connection with the Zeeman effect, as was at 

 first supposed. 



Radioactivity. The confidence with which the production of helium by the atomic 

 disintegration of radium is now regarded by physicists is shown by Strutt's 2 calculation 

 of the age of some specimens of thorianite by comparison with the observed evolution 

 of helium. He found that one specimen of thorianite was at least 280 million years 

 oki, a figure which should satisfy all geological requirements. 



A revised table of names of radioactive products has been put together by Ruther- 

 ford and Geiger. 3 Radium C gives rise both to Radium D and a new product called 

 Radium 2, which, however, starts a side branch of the radium family. Radium ema- 

 nation retains its somewhat clumsy name, although Ramsay proposes to re-name it 

 " Niton," from its intense luminosity when solidified. 



E. Regener 4 counts the number of a-rays and slow cathode ray particles ("5-rays ") 

 by allowing them to impinge upon a spray of fine oil-particles, which are then counted 

 in the field of an ultra-microscope. This is interesting in view of Boltwood and Ruther- 

 ford's definite proof that helium is monatomic, and each atom carries two elementary 

 positive charges. 



Chemical Analysis by Alpha-rays. The combined magnetic and electric deflection 

 of canal-rays (a-particles) gives rise to parabolic curves and other traces on a photo- 

 graphic plate which vary in a characteristic manner with the nature of the gas in the 

 vacuum tube. This observation has been utilised by Sir J. J. Thomson 5 to initiate a new 

 method of chemical analysis whose sensitiveness shall greatly exceed even the spectro- 

 scopic method. It has been found possible to trace the presence of monovalent hydro- 

 gen and oxygen, as well as carbon molecules of various atomicities and charges, but the 

 method is hardly ripe for general application yet, and the work of von Dechend, Ham- 

 mer, Koenigsberger, Kutschevski, Kilching, Fulcher, Gehrcke, Reichenheim, Baerwald 

 and Stark has not sufficed to clear up all the complex details of the behaviour of these 

 important forms of radiant matter. 



Photo electric Effects. The photo electric currents generated by the expulsion of 

 electrons from the illuminated surface (Hallwaehs), especially in the case of the alkali 

 metals and their amalgams, have acquired a new significance since they are being used 

 to discover selective absorption and atomic frequencies. Elster and Geitel, 6 by covering 

 the potassium cathode with a thin layer of colloidal potassium, succeeded in rendering 

 the cell highly sensitive up to a region of the spectrum far in the infra-red. A cell of 

 this kind would be suitable, as E. Bloch suggests, for use in a photophone. 



Among substances whose resistance is affected by light must now be counted sulphur 

 (Goldmann and Kalandyk); calcium sulphide (Vaillant); antimonite, both in the 

 native macrocrystalline state (Jaegers) and as pure artificial sulphide Sb>2 83 (Olie and 

 Kruyt); and even paraffin oil (Szivessy and Schaefer). It is significant that the action 

 of ultra-violet light on paraffin oil is in every respect similar to that of gamma-rays. 

 Probably we shall see this action before very long brought into a clear relation with 



1 Physikalische Zeitschrift, 12, p. i (1911). 



2 Royal Society, Proceedings, A84, p. 379 (1910). 

 : 3 Philosphical Magazine, 22, p. 621 (1911). 



4 Le Radium, 9, p. 150 (1912). 



b Ibid., 20, p. 752 (1910). 



6 Physikalische Zeitschrift, 12, p. 758 (1911). 



